The hot wire sensor comprises in a Wheatstone measuring bridge an electrically heated measuring wire and a compensation wire. The measuring bridge releases heat to a medium flowing past, for example, breathing gas, during the measuring operation. The higher the velocity of flow of the medium, the greater is the cooling of the measuring wire. In a manner basically known per se, the flow velocity of the medium in question and hence, for example, a breathing gas volume flow can be determined by means of the measuring bridge on the basis of the resulting temperature fluctuations and the changes in the electrical resistance of the measuring wire, which accompany these temperature fluctuations.
Especially the measuring wire is a part subject to wear of the hot wire sensor and requires replacement at regular intervals. A situation in which a high current flows through the measuring wire and the measuring wire is heated as a result to temperatures exceeding those occurring during the normal operation may occur during the replacement. This reduces the usability (service life) of the measuring wire or leads to a direct destruction of the measuring wire (“burnout”) already at the time of the replacement.
Various approaches have been taken to avoid this problem, but these require modifications of the apparatus in question, i.e., for example, of an anesthesia apparatus or ventilator. For example, a protective circuit or the like, whose function is specifically to avoid the overheating or burnout of the measuring wire, is installed within the framework of such a modification. Such modifications are uneconomical.